Before setting up a data connection to one or more base stations, the transmission clock and reception clock in each mobile radio receiver must be synchronized. This is generally achieved by means of a three-step method: slot synchronization (time slot synchronization) is carried out in a first synchronization step. Once the slot timings are known, the frame boundaries of the signal are determined in a second synchronization step (a frame synchronization). The scrambling code that is used by the transmitter (base station) is identified in a third synchronization step.
One of the major performance criteria for a synchronization apparatus is the detection probability (the probability of detecting a payload signal in the received signal), the false alarm rate (the probability of identifying a disturbance as a payload signal), the mean acquisition time and the time that the system requires in order to confirm negative detection (lack of synchronization). The detection probability should be as high as possible, the false alarm rate should be as low as possible, the mean acquisition time should be as short as possible, and the time to confirm negative detection should likewise be as short as possible.
The performance of a synchronization apparatus is significantly dependent on the frequency error that may exist between the transmitter and the receiver. In order to ensure optimum synchronization performance, the transmitter and the receiver must as far as possible operate with the frequencies accurately synchronized. If the mobile radio receiver has already been synchronized to a base station, then the frequency error with respect to a new cell to be looked for (for example, for a soft handover) is generally negligibly small (for example <0.1 ppm for UMTS). For initial synchronization attempts that, for example, have to be carried out after the mobile radio receiver has been switched on, on the other hand, there may well be a considerable frequency error between the transmitter (base station) and the mobile radio receiver. Typical frequency errors are in the region of 3 ppm. This leads to a considerable deterioration in the performance of the synchronization apparatus. The synchronization times are considerably longer, and in the worst case, the synchronization apparatus may fail completely.
A frequency error between the oscillator frequency of the transmitter (base station) and the oscillator frequency of the mobile radio receiver decreases the detection probability for a cell to be detected by the mobile radio receiver and, in consequence, the probability of the mobile radio receiver being able to successfully synchronize itself to this cell. In order to achieve a specific detection probability for a predetermined minimum received power level, two or more synchronization attempts, for example in series successively, are generally carried out until negative detection is signalled. It is normally necessary to search through a large number of frequency bands until cell synchronization is possible or negative detection can be confirmed. Thus, in practice, a compromise must be found between the achievable detection probability and the time period for negative detection. According to the prior art, there are two possible ways to increase the detection probability:    1. The number of synchronization attempts is increased. This increases the detection probability. However, this likewise increases the time period to confirm negative detection. Since it is frequently necessary to search through a large number of frequency bands before cell synchronization is possible, the increase in the number of synchronization attempts leads directly to the acquisition time being lengthened. In consequence, the improved detection probability is at the expense of, in some cases considerably longer acquisition times. The overall performance of the system is not improved in this case.    2. The frequency error between the transmitter and the mobile radio receiver is reduced, for example, by sampling a specific frequency band with a frequency grid that is as small as possible. For example, the step width of 6 ppm (maximum frequency error of 3 ppm) is reduced to a step width of 3 ppm (maximum frequency error of 1.5 ppm) with respect to the carrier frequency fc. Halving the maximum frequency error on the one hand considerably improves the detection probability, but on the other hand, also doubles the sampling time period for a predetermined frequency band, and thus the acquisition time. For example, a frequency band of 60 MHz around a carrier frequency of fc=2 GHz is available for UMTS (Universal Mobile Telecommunications System). About 5000 frequencies would have to be searched through for a step width of 6 ppm. Even if the mean search time were only 50 ms per frequency, doubling the number of frequencies to be searched through would incur an additional time penalty of about 250 s, and would thus considerably increase the mean acquisition time. Furthermore, the overall performance of the system is not significantly improved in this way.